It is known in the art that the particle size distribution of powdery solids is measured by utilizing light diffraction singly or diffraction and light scattering jointly each of which tends to occur when powdery solids afloat in dispersion is irradiated with laser beam. More specifically, the laser beam diffracts and scatters owing to the presence of powdery solids, and the spatial intensity distribution of the diffracting and scattering light is detected by photo-sensors such as ring detectors. The resulting data are converted into particle size distribution in terms of functions based on the Fraunhofer's diffraction theory or the Mie light scattering theory.
The known measuring devices necessarily uses a liquid dispersion agent so as to disperse the powdery solids, hereinafter referred to as "specimen", and the method using a liquid dispersing agent will be referred to as "wet measuring method". If the specimen is a type which tends to become dissolved or hardened at the presence of liquid, which is common with medicinal substances and cement, a liquid dispersing agent is substituted by air, hereinafter referred to as "dry measuring method".
In order to carry out the dry measuring method, a group of particles are obtained as a specimen from the powdery solids by dispersing them into a misty state (hereinafter called "aerosol particles") and the aerosol particles are irradiated with laser beam.
This known measuring method requires that the specimen has a constant concentration in a predetermined range.
The wet measuring method makes it easy to homogeneously disperse the specimen, and to maintain the concentration of it within a required range. The dry measuring method is difficult to keep the concentration of specimen in a particular range unless a special sampling device is employed. Even though a special sampling device is employed, the concentration of specimen is difficult to maintain because of unexpected factors such as unstable pneumatic pressure, improper handling of the sampling device and malfunction thereof. There are a kind of powders whose concentration is inherently unstable after they are ejected in aerosol, which results in imprecise measurement.